Recent studies from our laboratory and from others have added considerable strength to the oxidative modification hypothesis of atherosclerosis. There is now reasonably strong evidence that lipid peroxidation reactions occur in atherosclerotic lesions in animals and In humans, and that at least some of the LDL in atherosclerotic lesions is oxidatively modified. We have shown that probucol, a potent antioxidant, can slow the rate of degradation of LDL In foam cell-rich lesions of WHHL rabbits and, perhaps most importantly, slows the progression of atherosclerosis in these animals. We propose to focus our efforts on critically testing the oxidative modification hypothesis in vivo. Most directly, we will determine the effect of inhibitors of oxidative modification on the extent of atherosclerosis in WHHL and cholesterol-fed rabbits, by treatment with lipoxygenase inhibitors, treatment with a diet that alters LDL fatty acid composition, and treatment with antioxidants. If oxidative modification of LDL occurs In the artery, it should result in a more rapid uptake and degradation of the modified lipoprotein via scavenger receptors on macrophage foam cells. We propose studies to measure the rate of LDL degradation in atherosclerotic lesions in vivo, using the 'trapped label' method, to determine If Interventions which inhibit modification, such as those listed above, slow the rate of LDL degradation in lesions. Also, using competitive inhibition of scavenger receptors by, for example, maleylated albumin, we propose to quantify the contribution of scavenger receptor(s) in lesions in mediating the degradation of injected labeled native LDL. Our previous studies are consistent with the idea, but do not prove, that high concentration of LDL accumulates within the artery at lesion-susceptible sites, undergoes oxidation and generates a chemotactic stimulus for monocytes to penetrate into the subendothelial space. Using immunohistochemical, biochemical and molecular biology techniques, we will examine the natural history of the lesion to define when LDL modification may begin, which pro-oxidant enzymes may be present, with what cell types they are associated, whether inhibition of oxidative modification decreases the number of monocytes penetrating into the subendothelial space, etc. Finally, we have shown immunostaining of atherosclerotic lesions with antibodies against oxidation-specific epitopes on oxidized LDL and propose to characterize these lipid-protein adducts, specifically, if the staining in the extracellular matrix and adventitia are associated with lipoproteins, or are associated with specific matrix proteins.